UV - A, B, & C

Recently I have been trying to figure out if a UV filter is really needed on double-ended metal halide bulbs. Double-ended MH bulbs do not have an outer glass shell that other socket MH bulbs do. Double-ended MH bulb manufacturers recommend getting a UV shielding fixture, but is it necessary? UV-A runs about 310-400 nm and since it is lower on the spectum, it travels deeper in the ocean. What's protecting the corals from this UV light? Does UV-B come into play and is that what we are protecting them from? I talked to a friend of mine who is in the glass business and he said that there is no such thing as UV shielded glass. Are we being subject to marketting here by the manufacturers? Any ideas?

And all glass other then quartz will filter harmful UV (B&C and most of the shorter wavelength A) as far as I know.

Edited by jfinch

So - if all glass will filter harmful UV - what is all the concern about with HQI bulbs? The arc chamber is glass. Wouldnt that therefore filter out the UV?

I think the arc chamber may be quartz, not glass (silicon diozide) but this is just a guess...

Just think if she did we could take her on the road and make some money we could have much nicer tanks, just give it some thought.....

From Sarnacks link above:

Only reference I have right now is original "Korallrevs akvariet" by Fosså & Nilsen, vol 1. On page 46 there is a table that references OSRAM databook and it clearly shows that HQI lamps produce all types of UV radiation (A-C).

HQI 250W (at 1000 lumens):
UV-C 29 mW/m2
UV-B 108 mW/m2
UV-A 900 mW/m2

And from the http://www.srrb.noaa.gov/UV/ website:

UV wavelengths, particularly

those associated with UVB, are able to efficiently break DNA bonds. While UV,

particularly UVA, can assist in repairing DNA damage, the primary effect of UV appears

to be damaging to both DNA and individual cells.

UV is known to have primarily deleterious effects

on individual species, including phytoplankton, zooplankton, amphibians, terrestrial

plants and fish.

Marcus wrote: What's protecting the corals from this UV light?

This is a great question.  There are some very highly paid (we're talking millions here) people who are trying to find this out.  It is big big for cancer.  I don't have many answers but I do know some.  One thing is that the UV light actually does NOT penetrate as far as blue light.  Hard to believe but true.  The other item to note is that the water greatly absorbs (or disperses) the UV and is what protects the corals.  Also, corals near the surface slime coat themselves for protection.

But the big answer (again hard to believe) is that nothing is protecting the corals.  That is why they die.  Many coral experts think that corals would be growing at astonishing rates if there wasn't UV light.  It is hypothesised, and somewhat tested, that many corals are limited on how high up the water column they can grow simply because they are limited by UV. 

I think any liquid (if I remember correctly) will shield UV.  That is why a piece of glass is so important, it uses its liquid properties to shield UV and I'm convinced it works.

Adam

Marcus wrote: Adam, so you are saying that an acro that is 4" below the surface of the water will NOT be affected by UV because the water will protect it?

Oh no, just the opposite, I think it will be highly affected, which is why they don't naturally grow (well some do) that close to the water surface.  I think UV naturally is a huge threat to corals.  However at something like 30 feet, yes that water will have protected it.  Where inbetween 4" and 30' do I think it makes the cut off.... I don't know.  I'll guess at 10 feet. 

Hmm, I wonder how much UV our halides give off, compared to natural sunlight, and for how many hours per day, and stuff like that.

Adam

Jon and Sarnack, so if I buy a piece of tempered 1/8" "safety glass", it'll shield the UV-B&C?

I think it would be sufficient.  The outer tube on a single socketed bulb is thinner then that and it supposedly works.

Marcus - I could be wrong but here goes:
Tempered glass is a process of heating and cooling slowly to make it more resistant to shattering especially from rapid changes in temperature. Pyrex is a good example of this (the cooking glassware), and all labware glass. I dont think that tempering would have any effect on UV absorbtion.

UV Filtering glass has a metal film applied to it or laminated inside the glass.

But all glass has some UV filtering properties.

Edited by Sarnack